The majority of current concepts regarding reflex cardiovascular physiology derives from research conducted in anesthetized experimental animals and there are significant gaps in understanding of reflex cardiovascular control in man. This proposal comprises an interrelated series of experiments employing standard and new techniques to be conducted in conscious, unsedated man. The following aspects of reflex control will be studied: A. Studies of systemic arterial baroreceptor function: 1. Transfer characteristics of carotid sinus baroreceptors will be studied using neck suction in patients with complete heart block. 2. The interactions between different cardiovascular reflexes will be studied by determining the influence of systemic hypoxia and/or respiratory activity upon responses to neck suction. 3. The functional distribution of efferent cardiac baroreceptor nerves will be determined by stimulating left and right carotid sinus baroreceptors separately and measuring sinus node responses. 4. Responses of effector organs (A-V node, left ventricle and forearm resistance vessels) to arterial baroreceptor activation. B. Studies of left atrial receptors: Hemodynamic responses to distension of the pulmonary vein-left atrial junction.